This invention relates to Group III(a)--V(a) compound substrate surface preparation.
Substrate surface contamination prior to epitaxial growth can have deleterious effects on the structural, optical, and electrical properties of the finished device. Substrate surface cleanliness is especially important in molecular beam epitaxy (MBE) where, unlike other techniques such as liquid phase epitaxy or chemical vapor phase deposition, it is impossible to melt or etch back the substrate in situ prior to growth.
Ion sputtering has been used for the purpose of cleaning substrate surfaces, but this procedure should be controlled by a direct surface analysis technique such as Auger electron spectroscopy which is costly, complex, and time consuming. For commercial production it would be desirable to replace ion sputtering with a simpler technique such as heating to cause desorption of absorbed contaminants.
In a prior MBE technique taught by A. Y. Cho in U.S. Pat. No. 3,751,310 issued Aug. 7, 1973, (Case 2) and assigned to the assignee hereof and by Cho and Hayashi in the Journal of Applied Physics, Vol. 42, page 4222 (1971), the substrate was polished, mounted in a vacuum chamber, and heated. It was suggested that simply heating prior to growth was sufficient to remove atmospheric contaminants such as CO, from a GaAs substrate.
A more thorough technique as described by Cho and Hayashi, Solid State Electronics, Vol. 14, pages 125-132 (1971) involved first lapping the substrate on bromine-methanol soaked lens paper and then dipping it in bromine-methanol after which the substrate was rinsed with pure methanol and water. Substrates prepared in this manner exhibited surface defects which were attributed to such factors as oven contamination or faulty etching. However, the actual source of the problem was the atmospheric contamination, carbon-containing gases in particular, to which the substrate was exposed. This exposure resulted from rapid evaporation of the methanol between the etchant dip and the water rinse which would have formed a native oxide coating.